. ♪ >> reporter: the last time jennifer grey famously danced with a partner was 23 years ago in "dirty dancing" with the late bat trick swayze. >> the trust that i have to have with derek i had to have with patrick. they're very similar if the way they're both incredibly confident, natural dancers. i feel like -- now i'm going to get emotional -- oh, i feel like i have to do something that makes me really happy. ♪ >> reporter: only "good morning america" was invited to on-stage rehearsal. ♪ ♪ oh so closer now ♪ we are the champions of the world ♪ >> i'm convinced that i'm going to win because i'm so starving, i'm going to eat all the other contestants. >> reporter: former pro athletes, now, amateur dancers, claim they'll do almost whatever it takes to win. is there anything out here that you'll just like say, no way, i won't do it. you've seen guys take their shirts off. where do you draw the line on this? >> you know what right now, i know as we go along, the competition will get heated. ♪ i go crazy >> and fortunate to be around to where less clothing tends to happen, tha